Malaria is a tropical disease that is transmitted by Anopheles mosquitoes. Forty-one percent of the world's population lives in areas where malaria is transmitted. The global picture of malaria is grim: Worldwide, each year 300 million to 500 million people are infected and more than 1 million die, mostly children under the age of 5 (1). Although the vast majority of these cases are found in the 100 countries in the tropical regions of Africa, Asia, Central and South America where the disease is endemic, the recent increase in population movement to and from endemic areas through tourism and migration due to wars and socioeconomic factors has resulted in higher numbers of imported malaria cases where the disease is not endemic, such as the United States (2) and Europe (3-5). Malaria remains a major global health threat in the 21st century. The estimated cost of malaria in terms of strains on the health systems and economic activity lost is enormous. According to UNICEF, malaria costs Africa US$ 12 billions every year in lost productivity, reduced household income and expenditure on treatment. It slows economic growth by 1.3% per year.
Malaria is caused by protozoan parasites belonging to the genus Plasmodium. Four species of malaria parasites can infect humans: Plasmodium falciparum, P. vivax, P. oval and P. malariae. Species differentiation of Plasmodium is essential for selecting the proper treatment. Especially important is differentiating P. falciparum from the others since this species is responsible for 95% of deaths due to malaria (6). Malaria diagnosis, particularly in remote areas lacking laboratory support, frequently relies on the patient's symptoms. The first symptoms of malaria (fever, chills, sweats, headaches, muscle pains, nausea and vomiting) are not specific to malaria; clinicians often misdiagnose malaria infection. Symptomatic diagnosis is further complicated in highly endemic areas because a large proportion of the population can be infected but are not made ill by these parasites. Malaria morbidity, mortality and transmission can be reduced if infection can be promptly diagnosed and adequately treated.
Concerning the diagnostic procedure, the current standard method for diagnosis of malaria is the microscopic examination of Giemsa-stained thick and thin blood smears (7, 8). This procedure is time-consuming to prepare, read and interpret the slides. Previous studies have shown that even with experienced microscopists, misdiagnosis occurs, particularly in cases of mixed infection or low parasitemia (7, 9). Immunochromatographic assays based on antigen detection have been developed but are also relatively insensitive in cases of low parasitemia (10-12). In addition, antigenemia may persist weeks beyond the actual infection, leading to the false diagnosis of malaria parasitemia (10, 13). Molecular detection for Plasmodium diagnosis using the polymerase chain reaction (PCR) has resulted in increased sensitivity and species discrimination compared to either microscopic or immunochromatographic diagnosis of malaria (14, 15, 5, 16). However, most published PCR assays are gel based, resulting in a lengthy procedure not optimal for clinical use. Real-time PCR, a new methodology that employs fluorescent labels to enable the continuous monitoring of amplicon (PCR product) formation throughout the reaction has recently been adapted to detect all four human malaria parasites in blood samples (17-19). However, this procedure is laborious, costly and not suited for any laboratory interested in research related to malaria diagnosis.
Concerning the treatment, a limited number of drugs for treatment of malaria are available today. Due to worsening problems of drug resistance in many parts of the world, adequate treatment of malaria is becoming increasingly difficult. In the Central African Republic, the resistance of P. falciparum to chloroquine (CQ), the traditional first-line therapy for uncomplicated P. falciparum malaria, has been documented since 1983 (20) and the resistance to sulfadoxine-pyrimethamine (SP) since 1987 (21). The widespread resistance of P. falciparum to CQ and SP has also been found in sub-Saharan Africa (22) and on the north coast of Peru (23). Because of growing concerns about the development of resistance to antimalarial drugs when used alone, the affected countries are faced with the challenge of selecting a new first-line regimen and revising antimalarial treatment policies (24). Actually, the combination therapy is increasingly being regarded as the best strategy to improve efficacy and delay the development and spread of drug resistance (25). Evaluations of the efficacy of CQ+SP and amodiaquine (AQ)+SP in Bangui, Central African Republic (22), and SP+artesunate (AS) in Peru (23) for the treatment of uncomplicated P. falciparum malaria were performed. The obtained results suggest that the short-term efficacy of AQ+SP regimen is good, its long-term efficacy remains unknown (22). Fever and asexual parasite density decreased significantly and more rapidly in patients treated with SP+AS than in those who received SP alone. No severe adverse drug reactions were observed; however, self-limited rash and pruritis were significantly more common; and an exacerbation of nausea, vomiting, and abdominal pain were observed significantly and more frequently among patients who had received SP-AS combination therapy (23). Although some new drugs have appeared in the last 20 years (e.g., mefloquine, halofantrine, artemisinin derivatives, malarone), new, especially inexpensive and affordable drugs and more practical formulations of existing drugs/compounds are badly needed.
International efforts to combat malaria are also focused on the search for an effective and practical vaccine. There are four general categories of malaria vaccine candidates (26, 27), each representing a different stage of intervention. Virtually all the malaria vaccine candidates (with the exception of anti-disease vaccine described below) are cell surface antigens present during one of the three developmental stages of the Plasmodium parasite. 1/ Pre-erythrocytic (sporozoite) vaccines are those directed against the sporozoite (28, 29) and liver stages of the malaria parasite (30, 31). The sporozoite is the form of the parasite introduced into the human host by the bite of an infected mosquito which invades liver cells. A sporozoite vaccine could prevent infection either by blocking invasion of liver cells (antibody response) or destroying infected liver cells (cell-mediated response) by preventing release of parasites into the bloodstream. 2/ The asexual blood-stage (erythrocytic) vaccines (32-36) are directed against the merozoite stage of the parasite, which invades and replicates in the red blood cells. A blood-stage vaccine would be expected to reduce both the severity and duration of the disease by decreasing the blood parasite density, which correlates with reduced disease symptoms and risk of death. 3/ The transmission-blocking vaccines target the sexual stage of the parasite and are designed to raise antibodies (in humans) against the gamete stage of the parasite present in the mosquito gut (37, 38). Such antibodies taken up by a mosquito during a blood meal should block further parasite development in the mosquito, becoming a non-infectious vector. Blocking transmission of the parasite could reduce infectivity of the mosquitoes (carrying fewer parasites) and extend the useful life of a pre-erythrocytic or blood-stage vaccine by preventing transmission of antibody-resistant mutants. 4/ A fourth type of potential malaria vaccine is an anti-disease vaccine (39). This approach to a vaccine involves the identification of parasite toxins that contribute to the disease. An anti-disease vaccine is designed to prevent the anemia, coma, kidney disease and/or fever of malaria. Despite all efforts, none of the work in the above-mentioned four categories of malaria vaccine candidates has resulted in a practical vaccine at the present time.
Taking into account the above-mentioned problems, the present study is focused on P. falciparum species because this species is the major pathogen causing lethal malaria (95%) in man (6). The current situation of the clinical development to combat malaria shows that there is a need to design an easy, simple, and cost-effective procedure for the quantitative determination of P. falciparum DNA in malaria for quantitative diagnostic purpose and also for monitoring the efficacy of antimalarial treatment. To address such a need, a different approach is used for the development of a quantitative procedure for the determination of P. falciparum DNA in malaria. The new approach used in the current invention is different and contrary to previous approaches used for the development of malaria diagnostic tests because it involves the highly 42-kDa conserved C-terminal region of P. falciparum merozoite surface protein1 (MSP1) gene, in order to minimize all problems related to misdiagnosis of malaria, and because it involves the use of specific biotin label nucleotide probes directed to the 42-kDa C-terminal region of the MSP1 gene. Indeed, it is well known that the MSP1 is part of a complex that is thought to be involved in red blood cell invasion (40-43). Any research work for the development of a molecular diagnostic test, antimalarial drug or malaria vaccine that is based on the highly 42-kDa conserved C-terminal region of the MSP1 gene would consequently be useful to overcome all problems of misdiagnosis, drug resistance due to mutations in P. falciparum DNA. This new procedure entails the amplification of the 42-kDa C-terminal region of the MSP1 gene by using the PCR technique in the presence of digoxigenin-11-dUTP and the synthesis of the specific biotin label nucleotide probes directed to the 42-kDa C-terminal region of the MSP1 gene. These specific probes are then used in the Enzyme Linked Immunosorbent Assay (ELISA) for the quantitative determination of the 42-kDa C-terminal region of the MSP1 gene which leads to the quantitative determination of P. falciparum DNA in malaria for quantitative diagnostic purpose. The use of the specific probes allows the development of an easy and simple procedure for quantitative molecular diagnostic testing.